The possibility for biological terrorism has moved from the realm of speculation into reality. As a consequence, the National Institute of Allergy and Infectious Diseases has identified a number of agents that pose the greatest threat and has recommended that therapeutic agents be developed against them. Among that group of pathogens are smallpox and the viral hemorrhagic fevers (including the filoviruses Ebola and Marburg). In preparation for possible release of smallpox, plans are well underway to avail sufficient quantities of smallpox vaccine for a large number of people. However, within this population, there are individuals who will not be able to tolerate the vaccine or, for pre-existing health and/or age reasons, will not be suitable to receive vaccine protection. Thus, for these and other reasons, therapeutic agent development has been recommended by the NIAID. This proposal deals with two therapeutic goals in this direction: (i) agents for treating smallpox infection for instances where the vaccine was ineffective, not available or not administered for reasons just outlined, and (ii) drugs to counteract the effects of vaccination when administered. For the filoviruses Ebola and Marburg, which manifest themselves differently than smallpox but still produce horrendous effects, there are currently no pre-infection vaccines nor post-infection therapeutics available. To address these situations, plans are put forth for designing and developing drugs that would act by inhibiting viral replicative steps following infection by these viruses. Focus will be on nucleosides and nucleotides that affect, primarily, nucleic acid metabolism. A consortium of 3 chemists and 10 virologists has been assembled for this purpose. Because of the extensive collaborations that have been ongoing for some time by this group of chemists and virologists, it can be expected that promising anti-orthopoxvirus and anti-filovirus agents will be uncovered.